This invention relates to compositions which are curable to hydrophilic, water-absorbing silicone-organic copolymer elastomers via free-radical polymerization. The hydrophilic elastomers are useful as membranes for gas and other fluid separations.
Polydiorganosiloxane elastomers such as those which are predominantly polydimethylsiloxanes typically exhibit high permeability to various types of gases as compared with organic elastomers. Polydimethylsiloxane elastomers generally possess hydrophobic surfaces (i.e., water-in-air contact angles of greater than about 80.degree. at 25.degree. C. and more typically in the range of 95.degree.-110.degree.) due to the hydrophobic character of the polydimethylsiloxane chains forming the elastomer. As a result of their hydrophobic character, such elastomers tend to permit non-polar fluids and compounds to pass more readily through the elastomer than polar fluids such as water. It would be desirable to obtain an elastomer which possessed some of the high permeability characteristics of silicone elastomers, but which is hydrophilic and water-absorbing in nature, so as to more readily allow polar materials such as water, alcohols and polar bioactive agents such as insecticides or herbicides to pass through the elastomer. The hydrophilic character such as water-in-air contact angle and water absorbancy should be modifiable so as to enable one to adjust the rate at which a particular compound will permeate through the elastomer. If the hydrophilic, water-absorbing elastomer is to be used as a membrane for fluid separation, the elastomer should retain as much as possible of its original unhydrated physical properties such as tensile strength, elongation and tear strength after being hydrated and allowed to absorb water.
Attempts to provide such elastomers have been made in the past, particularly in the field of eye contact lenses where an oxygen permeable, soft, hydrophilic elastomeric material is desirable. U.S. Pat. No. 4,136,250 to Mueller, et al. (issued 1/23/79) and hereby incorporated by reference provides a water-insoluble hydrophilic gel comprising about 20 to 90% by weight of (1) a hydrophilic (a) polymer of identical or different water-soluble monoolefinic monomers or (b) copolymer of said water-soluble monomers with 1 to 80% (of total monomers) of water insoluble, identical or different monoolefinic monomers; ingredient (1) is cross-linked with (2) about 10 to 80% by weight of a terminal polyolefinic siloxane macromer having a molecular weight of from about 400 to about 8500 to form a gel. Unlike the curable composition and hydrophilic elastomers of the present invention, Mueller, et al. teach that the siloxane macromer is the hydrophobic portion of the gel product and provides flexible cross-links and improved oxygen permeability. A water soluble monoolefinic monomer is required to be present as at least 20% by weight of the total monoolefinic monomers used to form the Mueller, et al. hydrogel. As will be described, we have discovered that the polysiloxane component of certain compositions can act as the hydrophilic portion within certain limits and have therefore found that substantially water insoluble aliphatically unsaturated monomers can be used to the exclusion of water soluble monoolefinic monomers to obtain hydrophilic, water-absorbing silicone elastomers which are oxygen permeable.
U.S. Pat. No. 4,235,985 to Tanaka, et al. (issued 11/25/80) teaches copolymers for contact lenses which are a copolymer of a organosiloxane monomer which contains a pendant hydroxyl radical for hydrophilicity and may optionally contain a polyether group (to improve the hydrophilicity of the copolymer) and a hydrophobic methacrylic acid alkyl ester. The copolymer is said to be hydrophilic, but is hard and is substantially non-water absorptive unlike the water-absorbing elastomers of the present invention. The organosiloxane monomer is employed to provide both oxygen permeability and hydrophilicity, but differs in structure from those employed in the present invention.
U.S. Pat. No. 4,260,725 to Keogh, et al. (issued 4/7/81) teaches a water-absorbing, soft, hydrophilic, flexible contact lens which is oxygen permeable. It teaches a copolymer of organic monomers which may or may not have hydrophilic groups such as hydroxyl groups present therein with a polysiloxane which is alpha, omega-terminally bonded through divalent hydrocarbon groups to polymerizably activated unsaturated groups and which polysiloxane has hydrophilic sidechains. Keogh, et al. fails to teach the polysiloxanes employed in the present invention. When polyether sidechains are used by Keogh, et al., they prefer methoxy end-capped polyether sidechains. This differs from the aliphatically unsaturated terminal groups employed in the present invention which enable the entire polysiloxane block copolymer to form the elastomer and thereby contribute to the retention of physical properties after absorbing water. Furthermore, Keogh, et al. make no distinction between the use of water soluble monomers such as 2-hydroxyethylmethacrylate versus substantially water insoluble organic comonomers such as methyl methacrylate in their compositions while the present invention employs substantially water insoluble organic monomers. These latter monomers are employed in the present invention to obtain cured elastomers with desirable physical strength and resistance to tearing after absorption of water.